The following abbreviations are herewith defined, at least some of which are referred to within the following description of the prior art and the present invention.
3GPP 3rd Generation Partnership Project
AAA Authentication, Authorization and Accounting
A-GW Access Gateway
AGW-UP Access Gateway User Plane (serving GW plus PDN GW)
AMR Adaptive Multi Rate
AP Access Point
BSC Base Station Controller
BSS Base Station Subsystem
BSSAP Base Station Subsystem Application Part
CBC Cell Broadcast Centre
CC Call Control
CGI Cell Global Identity
CM Connection Management
CN Core Network
CS Circuit Switched
CSR Circuit Switched Resources
DNS Domain Name Service/Server
DTM Dual Transfer Mode
EPC Evolved Packet Core
ESP Encapsulating Security Payload
E-UTRAN Evolved-UMTS Radio Access Network
FQDN Fully Qualified Domain Name
GA Generic Access
GA-CSR Generic Access-Circuit Switched Resources
GAN Generic Access Network
GANC Generic Access Network Controller
GERAN GSM EDGE Radio Access Network
GPRS General Packet Radio Service
GSM Global System for Mobile communications
GTP GPRS Tunneling Protocol
HLR Home Location Register
HSS Home Subscriber Server
IMS IP Multimedia Subsystem
IMSI International Mobile Subscriber Identity
IP Internet Protocol
IWU Interworking Unit
LAI Location Area Identity
LTE Long-Term Evolution
MAC Medium Access Control
MGW Media GateWay
MM Mobility Management
MME Mobile Management Entity
MS Mobile Station
MSC Mobile Switching Centre
MTP Message Transfer Part
OFDM Orthogonal Frequency Division Multiplex
PCRF Policy and Charging Rules Function
PCSC Packet Circuit Switch Controller
PDCP Packet Data Convergence Protocol
P-GW Packet-Gateway
PDN-GW Packet Data Network-Gateway
PMSC Packet MSC
PS Packet Switched
PSS Packet Switched Services
PSTN Public Switched Telephone Network
RAN Radio Access Network
RANAP Radio Access Network Application Part
RC Resource Control
RLC Radio Link Control
RTP Real Time Protocol
SCCP Signalling Connection Control Part
SAE System Architecture Evolution
SC-FDMA Single Carrier-Frequency Division Multiple Access
SEGW Security Gateway
SGSN Serving GPRS Support Node
S-GW Serving Gateway
SMLC Serving Mobile Location Centre
SMS Short Message Service
SRVCC Single Radio Voice Call Continuity
SS Supplementary Service
TA Tracking Area
TAU Tracking Area Update
TCP Transmission Control Protocol
UDP User Datagram Protocol
UE User Equipment
UMA Unlicensed Mobile Access
UMTS Universal Mobile Telecommunications System
UTRAN UMTS Radio Access Network
WCDMA Wideband Code Division Multiple Access
The present invention described herein relates to a solution called CS-over-LTE-via-GAN (CSoLTEvGAN, also called Voice over LTE Generic Access (VoLGA)) and a handover from CSoLTEvGAN towards GERAN/UTRAN. Hence, a brief description is provided next about the current state of the art associated with CSoLTEvGAN and the CSoLTEvGAN to GERAN/UTRAN handover procedure. This brief description has been divided into several sections as follows:
CSoLTE (Circuit Switched Services over LTE) background and the different possibilities for CSoLTE solutions.
GAN background.
CSoLTEvGAN background.
Principles for handover from CSoLTEvGAN towards GERAN/UTRAN.
Problems with existing solutions.
CSoLTE Background
Mobile CS services based on GSM and WCDMA radio access are a world-wide success story and allow a user with a single subscription to obtain telecommunication services in almost all countries of the world. Also today, the number of CS subscribers is still growing rapidly, boosted by the roll-out of mobile CS services in densely populated countries such as India and China. This success story is furthermore extended by the evolution of the classical MSC architecture into a softswitch solution which allows the use of a packet transport infrastructure for mobile CS services.
Recently the 3GPP work item “Evolved UTRA and UTRAN” (started in summer 2006) defined a Long-Term Evolution (LTE) concept that assures the competitiveness of 3GPP-based access technology. The LTE concept was preceded by an extensive evaluation phase of possible features and techniques in RAN workgroups which concluded that the agreed system concepts can meet most of the requirements and that no significant issue was identified in terms of feasibility. The LTE will use OFDM radio technology in the downlink and SC-FDMA for the uplink, allowing at least 100 Mbps peak data rate for downlink communications and 50 Mbps peak data rate for uplink communications. LTE radio can operate in different frequency bands and is therefore very flexible for deployment in different regions of the world.
In parallel to the RAN standardization, the 3GPP also supported a System Architecture Evolution (SAE) work item to develop an evolved packet core network. The resulting SAE core network is made up of core nodes, which are further, split, into a Control Plane mode (MME), a User Plane node (S-GW), and Packet Data Network GW (PDN GW or P-GW). In this document, a co-location of S-GW and P-GW is denoted as an Access GW (AGW). FIG. 1 (PRIOR ART) is a block diagram illustrating a LTE/SAE architecture according to 3GPP TS 23.401 V8.4.1 (the contents of which are incorporated by reference herein).
The LTE/SAE architecture has been specified such that only a Packet Switched (PS) domain will be supported, i.e. all services are to be supported via this PS domain. However, GSM (via DTM) and WCDMA provide both PS and CS access simultaneously. So, if telephony services are to be deployed over LTE radio access, an IMS based service engine is mandatory. It has been investigated how to use the LTE/SAE architecture as the access technology to the existing CS core domain infrastructure. The investigated solutions are called “CS over LTE” solutions and the basic architecture for these solutions is shown in the block diagram of FIG. 2 (PRIOR ART).
In the CSoverLTE architecture, a Packet MSC (PMSC) can serve both traditional 2G and 3G RANs plus the new CS (domain) over the LTE based solutions. The PMSC contains two new logical functions which are called Packet CS Controller (PCSC) and Interworking Unit (IWU) both of which are described next with respect to FIG. 3 (PRIOR ART).
The communication between the terminal (MS) and the PMSC is based on the SGi interface. This means that all direct communication between the terminal and the PCSC and the IWU in the PMSC is based on IP protocols and that the MS is visible and reachable using an IP-address via the AGW. This communication is further divided into two different interfaces, U8c for the control plane and U8u for the user plane. FIG. 4 (PRIOR ART) shows the CSoLTE control plane architecture (i.e., the U8c interface) between the terminal and the PMSC. FIG. 5 (PRIOR ART) shows the CSoLTE user plane architecture (i.e., the U8u interface) between the terminal and the PMSC. The PCSC has also an Rx interface to the PCRF for allocation of the LTE/SAE bearers (see FIG. 3).
Three different solutions for providing CSoLTE service have been identified so far. The first solution is called “CS Fallback” where the terminal performs SAE MM procedures towards the MME while camping on LTE access. The MME registers the terminal within the MSC-S for CS based services. Then, when a page for CS services is received in the MSC-S it is forwarded to the terminal via the MME and the terminal performs fallback to the 2G or 3G RANs. Similar behavior applies for terminal originated CS services and when these are triggered and the terminal is camping on LTE access, it will fallback to 2G or 3G RANs and trigger the initiation of the CS service. This solution has been described in co-assigned U.S. patent application Ser. No. 12/531,651 and specified in 3GPP TS 23.272 V8.2.0 (the contents of both documents are incorporated by reference herein).
The second solution is called CS over LTE Integrated (CSoLTE-I). In this solution the same SAE MM procedures as for “CS Fallback” are used, but instead of performing a fallback to the 2G or 3G RANs, the terminal performs all the CS services over the LTE access. This means that the CS services (also called Connection Management, CM, procedures) are transported over IP-based protocols between the PMSC and the terminal using the LTE access and the SAE nodes like AGW-UP.
The third solution is called CS over LTE Decoupled (CSoLTE-D). In this case both MM and CM procedures are transported over IP-based protocols directly between the PMSC and the terminal using the LTE access and the SAE user plane nodes like the AGW-UP. This solution has been described in the co-assigned U.S. patent application Ser. No. 12/522,408 (the contents of which are incorporated by reference herein).
GAN Background
3GPP has standardized the Generic Access Network (GAN)-concept starting from 3GPP Release-6. The correct name is “Generic Access to A/Gb Interfaces” and this standardization was based on the Unlicensed Mobile Access (UMA) de-facto specifications.
GAN provides a new Radio Access Network (RAN) and the node corresponding to GERAN BSC is called a Generic Access Network Controller (GANC). GAN is specified in the 3GPP TS 43.318 V8.3.0 and 3GPP TS 44.318 V8.4.0 (the contents of both documents are incorporated by reference herein). FIG. 6 (PRIOR ART) shows the functional architecture of GAN as indicated in 3GPP TS 43.318. The basic principle is that the MS/UE is a dual-mode, dual radio handset including for example both WiFi and 3GPP-macro radio support (GSM, WCDMA or both). For instance, the MS connects to a WiFi Access point (AP) (not shown) using the WiFi Radio. The GAN standard defines how the MS can function in GAN mode and access the services provided by the GSM CN (Core Network) using the Up-interface between the MS and the GANC.
The initial GAN standard has been called “2G-GAN” or “GSM-GAN” as the standard GSM interfaces, A and Gb are used between the GANC and the CN. In addition, work is currently ongoing to standardize a “3G-GAN” or “WCDMA-GAN” solution. In this case, the GANC will use the standard WCDMA interfaces, for example the Iu-cs and the Iu-ps interfaces to connect to the CN. The resulting standard could be called “Generic Access to Iu interface” or the shorter term “GAN-Iu”.
FIG. 7 (PRIOR ART) has been provided to show the CS Domain Control Plane Architecture related to GAN and the Up-interface. The main principle is that the GANC uses the normal A-interface signaling towards the MSC and interworks the related protocol, like BSSAP, towards the relevant GAN-protocols, like GA-CSR (Generic Access, Circuit Switched Resources), in both directions.
CSoLTEvGAN Background
The CSoLTEvGAN solution has not yet been standardized but exists as one of the alternatives for CS service support over the LTE described in 3GPP TR 23.879 V1.1.1 (the contents of which are incorporated herein). The basic idea for the CSoLTE alternative is to see LTE as a Generic Access Network and use the GAN protocols for the control and user planes. FIG. 8 (PRIOR ART) is a diagram illustrating the CSoLTEvGAN architecture.
One major difference for the GANC in this situation when compared to the aforementioned GAN solution is that the handover is triggered using the SRVCC (Single Radio Voice Call Continuity) procedure over the Sv′ interface (see FIG. 9). The SRVCC procedure is a procedure in 3GPP to switch an IMS-anchored voice call in LTE to the CS domain (MSCs) in GSM or WCDMA. Only the handover part is used from the SRVCC solution as there is no need for any IMS Session transfer procedure. The SRVCC procedure has been standardized in 3GPP TS 23.216 V8.2.0 (the contents of which are incorporated by reference herein).
There are different ways to select a GANC for the UE (i.e. for GAN registration). One possibility is the EPC-based selection i.e. that the GANC is selected for the UE based on the LTE/SAE cells (E-CGI) and tracking areas (TA) during attach or Tracking area update (TAU) (e.g. that the MME returns the GANC address information to the UE). Still another possibility is that the GANC selects the correct GANC based on the E-CGI and TAI of the current LTE-cell as reported by the UE and then redirects the UE to the correct GANC. In all the different variants, the GANC selection can be DNS-based load balancing in a specific area. This means that a pool of GANCs is serving the whole or parts of the LTE/SAE network and the DNS provides the means to divide the different UEs to different GANCs.
Principles for Handover From Csoltevgan Towards GERAN/UTRAN
The main principle for handover from CSoLTEvGAN towards GERAN/UTRAN is that parts of Single Radio Voice Call Continuity (SRVCC) are used as the handover trigger for handover from CSoLTEvGAN towards GERAN/UTRAN. This means that the MME will trigger the handover as in SRVCC. However, instead of contacting the MSC over the Sv-interface, the MME will contact the GANC over Sv′ interface (see FIG. 8). So the Sv′ interface between the MME and the GANC is used to support the Handover from CSoLTEvGAN to GERAN/UTRAN procedure. This interface is denoted as Sv′ since it is a reference point between MME and GANC and may only implement parts of the current version of the standardized Sv reference point, where Sv is between the MME and MSC. FIG. 9 (PRIOR ART) shows the Sv′ interface between the MME and GANC where the Sv′ interface is implemented using the protocol GTP.
FIGS. 10A-10B (PRIOR ART) shows the steps for implementing the Handover from CSoLTEvGAN to GERAN/UTRAN procedure. The main part with respect to this particular discussion is that for this handover procedure the Source MME selects the GANC based on the target cell information i.e. the LAI/CGI of the target GERAN/UTRAN cell associated with the moving UE. However, the selected GANC may not be the originally registered GANC which leads to a handover problem as discussed in detail next.
Problems with Existing Solutions
A problem exists because of how the MME selects a GANC during the GAN Registration procedure when the UE is in a LTE cell and how the MME selects a GANC during a CSoLTEvGAN handover procedure when the UE moves from the LTE cell to a GERAN/UTRAN cell. As discussed above, during the GAN registration the MME selects a GANC for the UE based on LTE/SAE cells and/or Tracking Areas plus DNS based load balancing may be used. Then during the handover, the MME selects a GANC based on the target GERAN/UTRAN cell identifier or LAI. The main problem is that the handover request (from CSoLTEvGAN to GERAN/UTRAN procedure) from the MME needs to be addressed to the same GANC that was selected for the UE during GAN Registration. Thus, if the MME selects the wrong GANC during the SRVCC handover procedure then the MME will send the handover request to the wrong GANC. If this occurs, then the Handover procedure will fail which will most likely lead to the dropping of calls and unsatisfied customers.
FIG. 11 (PRIOR ART) is an exemplary network scenario which has been provided to further highlight this particular problem. A UE is shown in the LTE cell E-CGI-3 belonging to TA with TAI-1. GANC1, GANC2 or GANC3 can be selected by the UE at registration based on the LTE cell and TA and load balancing between GANCs. In this example, assume the UE at registration selects GANC2. If the UE enters CS active state via GANC2 and MSC1 and then later on moves towards GERAN coverage shown with the GSM cell CGI-4 belonging to LA with LAI-1, then the CSoLTEvGAN handover procedure would be triggered (i.e. the one based on SRVCC). In this case, the source MME would need to select the correct GANC, but the information for the GANC selection in the Source MME is only the GSM Target cell i.e. CGI-4 and LAI-1. This is not enough information for the MME to know that it should select GANC2 in this case. Accordingly, there is a need to address this problem to ensure that the handover will not fail. Plus, there is a need to address this problem without introducing any changes or at least minimizing changes to the MME. These needs and other needs have been satisfied by the present invention.
EP-A-2043378 discloses a method for controlling registration of an MS and a Generic Access Network Controller (GANC). The method includes a GANC receiving a registration message from an MS, when there is an ongoing service between the GANC and the MS, the GANC triggers a handover procedure, or the GANC does not respond to the registration message. The GANC includes a receiving unit, a transmitting unit, a determining unit adapted to determine whether a registration request message should be redirected according to current network condition and registration information of an MS, and a controlling unit adapted to instruct the message transmitting unit, according to the determined result to send a notification message to the MS instructing MS's corresponding operation.
WO-A-2008081310 discloses a method that includes initiating a handover procedure during an ongoing call of a wireless user terminal in one of a circuit switched domain or a packet switched domain, setting parameters allowing the other domain to determine the actual resources needed to continue the call in the other domain, sending an indication of these parameters towards a network element of the other domain and completing the handover procedure. When the ongoing call is in the circuit switched domain completing the handover procedure results in handing over the ongoing call to the packet switched domain, and when the ongoing call is in the packet switched domain completing the handover procedure results in handing over the ongoing call to the circuit switched domain. For example, the circuit switched domain may be a GERAN network and the packet switched domain may be an E-UTRAN (LTE) network. In the GERAN network the handover procedure may be accomplished at least in part through a Gs interface between a mobile switching center and a serving general packet radio system support node and/or through a Gb interface between a base station system and the serving general packet radio system support node. The handover procedure is accomplished at least in part by signaling conducted over an S3 interface between the serving general packet radio system support node of the GERAN network and the mobility management entity of the E-UTRAN network.